Abstract

The photospheric magnetic field is the source of the coronal and heliospheric magnetic fields (HMF), but their mutual correspondence is non-trivial and depends on the phase of the solar cycle. The photospheric field during the HMF sector crossings observed at 1 AU has been found to contain enhanced field intensities and definite polarity ordering, forming regions called Hale boundaries. Here we separately study the structure of the photospheric field during the HMF sector crossings during Solar Cycles 21 – 24 for the four phases of each solar cycle. We use a refined version of Svalgaard’s list of major HMF sector crossings, mapped to the Sun using the solar wind speed observed at Earth, and the daily level-3 magnetograms of the photospheric field measured at the Wilcox Solar Observatory in 1976 – 2016. We find that the structure of the photospheric field corresponding to the HMF sector crossings and the existence and properties of the corresponding Hale bipolar regions varies significantly with solar cycle, solar cycle phase, and hemisphere. The Hale boundaries in more than half of the ascending, maximum, and declining phases are clear and statistically significant. The clearest Hale boundaries are found during the (+,−) HMF crossings in the northern hemisphere of odd Cycles 21 and 23, but less systematical during the (+,−) crossings in the southern hemisphere of even Cycles 22 and 24. No similar difference between odd and even cycles is found for the (−,+) crossings. This shows that the northern hemisphere has a more organized Hale pattern overall. The photospheric field distribution also depicts a larger area for the field of the northern hemisphere during the declining and minimum phases, in a good agreement with the bashful ballerina phenomenon.

We acknowledge the financial support by the Academy of Finland to the ReSoLVE Centre of Excellence (project no. 272157). The Wilcox Solar Observatory data used in this study were obtained via the web site http://wso.stanford.edu courtesy of J.T. Hoeksema. The OMNI data were obtained from the GSFC/SPDF OMNIWeb interface at http://omniweb.gsfc.nasa.gov . We thank L. Svalgaard for the HMF polarity data. We are also grateful to the National Geophysical Data Center and the WDC-SILSO, Royal Observatory of Belgium, Brussels, for the sunspot data.